Spray Equipment and Application Technology

Spray Equipment and Application Technology

Fundamentals of Spraying

Spraying is the process of transforming a liquid (primer, base coat, clear coat) into fine droplets that are projected onto a surface. The quality of the finish depends on three interdependent factors: atomization, deposition, and wetting. Atomization is the breakdown of liquid into fine particles; it is controlled by air pressure, fluid flow rate, and nozzle design. Poor atomization produces a grainy film or an orange peel effect.

Solids transfer (or transfer efficiency) is the percentage of paint that actually adheres to the part compared to the total amount sprayed. Conventional spray guns have a transfer efficiency of 25–40%, HVLP (High Volume Low Pressure) guns achieve 65–85%, and electrostatic systems reach 70–90%. Health Canada's Volatile Organic Compound (VOC) Emissions Regulations require shops to use high-transfer-efficiency equipment for automotive refinishing applications.

Types of Spray Guns

The HVLP gun is the most common in body shops. Its principle: a large volume of air at low pressure atomizes the paint. The maximum pressure at the nozzle must not exceed 10 psi (69 kPa). This limitation reduces bounce-back and overspray, improving transfer efficiency and reducing VOC emissions. The disadvantage: a slower flow rate and increased sensitivity to product viscosity.

The conventional gun uses high air pressure (30–50 psi) for atomization. It produces a very smooth finish but with low transfer efficiency. It is reserved for localized touch-ups or high-viscosity products.

The LVLP gun combines low air volume (approximately 10–15 CFM) with low pressure. It is ideal for shops with small-capacity compressors. Its transfer efficiency is slightly lower than HVLP.

Gun Components and Their Function

The gun body contains the air passage, fluid passage, and trigger mechanism. The trigger is two-stage: the first pressure opens the air passage, additional pressure opens the fluid needle. This sequence allows you to purge the gun before application.

The fluid tip (or nozzle) determines the paint flow rate. Common sizes range from 1.2 mm to 2.0 mm. A 1.3–1.4 mm nozzle is standard for base coats and clear coats; 1.5–1.8 mm for primers; 2.0 mm for fillers and thick products. The air cap (or atomization head) controls the spray pattern shape. The air cap orifices produce a round pattern (for touch-ups) or a fan pattern (for large surfaces). Fan adjustment is made with the top knob, fluid flow with the bottom knob.

The needle and seat form the seal for the fluid passage. A worn seat or damaged needle causes spitting and runs. The cup can be top-mounted (gravity) or bottom-mounted (siphon). The top-mounted cup offers better transfer and is preferred for metallic base coats, as it avoids flow variations. The bottom-mounted cup is used for large volumes.

Gun Adjustments and Application Parameters

Inlet Pressure and Nozzle Pressure

Inlet pressure (or trigger pressure) is measured at the gun inlet with a digital gauge or built-in regulator. For an HVLP, typical inlet pressure is 25–30 psi (172–207 kPa) to achieve 8–10 psi at the nozzle. The pressure drop depends on air flow rate and internal design. A nozzle gauge (or test air cap) is required to verify actual pressure.

The formula for effective nozzle pressure for an HVLP is approximate: P_nozzle = P_inlet − (ΔP_due to internal restrictions). In practice, you use a pressure regulator with a gauge built into the gun. Adjustment is made with the gun in hand, trigger fully depressed, without fluid.

Viscosity and Thinning

Viscosity is a liquid's resistance to flow. It is measured with a viscosity cup (Ford #4 cup or Zahn cup). The procedure: fill the cup, time the complete flow. Typical values for spraying: base coat 18–22 seconds, clear coat 20–25 seconds, primer 25–30 seconds (at 20 °C). Viscosity that is too high produces a grainy film; too low causes runs and poor opacity.

Product temperature influences viscosity. The general rule: a 10 °C increase reduces viscosity by approximately 50%. You must therefore adjust thinning according to ambient temperature. The thinner must be compatible with the resin system (for example, solvent-based thinner for solvent-based products, deionized water for water-based products).

Spray Distance and Travel Speed

The distance between the nozzle and the surface should be 15–25 cm for HVLP, 20–30 cm for conventional. A distance that is too short causes runs and excess material; too long produces a dry, grainy film (dry spray). Travel speed must be constant, approximately 30–60 cm per second. Irregular speed produces variations in film thickness.

Overlap of passes should be 50% for metallic base coats (to ensure uniform flake distribution) and 50–75% for clear coats. Each pass must start and end off the part (triggering the gun before the surface and releasing after).

Cup Pressure and Fluid Flow

Fluid flow is controlled by the needle adjustment screw. Flow that is too high produces runs; too low produces a dry film. The rule: open the fluid screw until the pattern is complete and regular, without excessive overspray. For siphon-feed guns, cup pressure must be 5–10 psi (34–69 kPa) to force fluid toward the nozzle.

Nozzle Types and Air Caps

Nozzle Numbering

Nozzles are identified by a two-digit code. The first digit (multiplied by 0.1 mm) indicates the orifice diameter. For example, a "13" nozzle has a diameter of 1.3 mm. The second digit indicates the pattern shape: 1 = round pattern, 2 = medium fan, 3 = wide fan. An air cap "13-2" has a 1.3 mm orifice and a medium fan.

Air Caps and Spray Patterns

The standard air cap produces a fan pattern. The round-pattern cap is used for localized touch-ups (less than 5 cm in diameter). The wide-fan cap is used for large flat surfaces. The orientation of the air cap determines the orientation of the fan: vertical for horizontal passes, horizontal for vertical passes.

Equipment Maintenance and Cleaning

Daily Cleaning

After each use, the gun must be cleaned immediately. The procedure: empty the cup, rinse with the appropriate solvent, spray clean solvent through the gun, disassemble the air cap and nozzle, clean the orifices with a brass brush (never steel), and dry with compressed air. Never immerse the complete gun in solvent, as this damages seals and packings.

Periodic Maintenance

Every 40–50 hours of use, check the needle and seat for wear. A simple test: fill the cup with solvent, close the trigger, observe if fluid flows out. A leak indicates a worn seat. Lubricate the trigger pivot points with a silicone lubricant (never oil, which contaminates paint). Check the tightness of air fittings.

Compressor and Air Filtration

The compressor must supply sufficient air flow: an HVLP gun consumes 8–15 CFM (cubic feet per minute) at 30 psi. The compressor tank should have a capacity of at least 60 litres to avoid short cycling. Compressed air must be filtered to remove water, oil, and particles. A water separator and coalescing filter are mandatory. CSA B149.1 (Natural Gas and Propane Installation Code) applies to dryers and booth heating systems, but spray air filtration falls under shop best practices and occupational health and safety requirements.

Spray Booths and Ventilation

Safety Requirements

Spray booths must comply with the Canadian Electrical Code, Part V (C22.1) for electrical installations in classified areas. Rule 8-200 of the Canadian Electrical Code, Part I, applies to branch circuits in hazardous locations. Booths must be classified as Class I, Division 1 for the interior of the booth (presence of flammable vapors) and Class I, Division 2 for adjacent areas. All electrical equipment (lighting, motors, fans) must be certified for these areas.

Ventilation must maintain a minimum air velocity of 0.5 m/s (100 ft/min) through booth openings during spraying. Air flow rate is calculated by: Q = V × A, where Q is the flow rate (m³/s), V the velocity (m/s), A the area of openings (m²). For example, a booth with an opening of 3 m × 2.5 m (7.5 m²) requires a flow rate of 0.5 × 7.5 = 3.75 m³/s (13,500 m³/h).

Booth Filters

Intake filters (pre-filters and final filters) must be replaced when pressure drop exceeds 125 Pa (0.5 inches of water column). Exhaust filters (paint filters) must be replaced when saturated. A differential manometer is mandatory to monitor filter condition. NFPA 33 (adopted by reference in Canada) requires that filters be made of non-combustible or slow-burning material.

Essential Calculations and Conversions

Wet and Dry Film Thickness

Wet film thickness (WFT) is related to dry film thickness (DFT) by the volume solids percentage (VS): DFT = WFT × VS / 100. For example, a clear coat with 45% volume solids applied at a WFT of 100 µm will produce a DFT of 45 µm. To achieve a DFT of 50 µm with a product at 40% VS, the required WFT is: WFT = 50 × 100 / 40 = 125 µm.

Theoretical Coverage

Theoretical coverage (in m²/L) is: Coverage = VS × 10 / DFT (in µm). For a product at 50% VS and a DFT of 50 µm: Coverage = 50 × 10 / 50 = 10 m²/L. In practice, you must apply a loss factor of 20–30% (overspray, waste).

Pressure Conversion

The conversion: 1 psi = 6.895 kPa. To convert psi to kPa, multiply by 6.895. To convert kPa to psi, divide by 6.895.

Spray Defects and Correction

Orange Peel

Orange peel is a textured appearance on the surface, caused by insufficient atomization, excessive distance, viscosity that is too high, or air pressure that is too low. Correction: reduce the distance, increase air pressure (within HVLP limits), thin the product further, or use a smaller nozzle.

Runs and Sags

Runs are vertical flows of paint, caused by flow that is too high, distance that is too short, travel speed that is too slow, or viscosity that is too low. Correction: reduce fluid flow, increase the distance, speed up the movement, or thicken the product.

Dry Spray

Dry spray produces a grainy, powdery film, caused by excessive distance, air pressure that is too high, ambient temperature that is too high, or solvent that evaporates too quickly. Correction: move the gun closer, reduce pressure, use a slower thinner.

Spitting

Spitting is an irregular spray pattern, caused by a partially blocked nozzle, an air leak in the fluid passage, or low fluid level in the cup. Correction: clean the nozzle, check the seals, fill the cup.

Advanced Application Techniques

Applying Metallic Base Coats

Metallic (flake) base coats require a specific technique. The first coat is applied in a cross pattern (horizontal then vertical passes) to even out the flakes. Subsequent coats are applied in horizontal passes only, with 50% overlap. The final coat is applied "dry" (slightly greater distance, slightly lower pressure) to lock in the flakes. Never sand a metallic base coat before applying clear coat.

Applying Clear Coats

Clear coat is applied in two full coats with 75% overlap. The first coat is light (a mist coat) to ensure adhesion. The second coat is full, at a distance of 15–20 cm. Flash time between coats must be respected according to the manufacturer's instructions (generally 5–10 minutes). Surface temperature should be 18–25 °C for optimal curing.

Spot Repair

The localized touch-up technique involves applying paint only to the damaged area, with a gradual blend. Use a round pattern and reduced pressure. The repair area must be prepared with primer and sanded with fine grit (P800 to P1000). Clear coat is applied slightly overlapping the adjacent area, then sanded and polished after curing.

Safety and Regulations

Respiratory Protection

Solvent vapors and isocyanates (present in clear coat hardeners) are extremely toxic. Wearing a supplied-air respirator (SAR) is mandatory when spraying products containing isocyanates. Cartridge filter masks are not sufficient for isocyanates. CSA Z94.4 (Selection, Use, and Care of Respirators) applies. Supplied air must be of respirable quality (Grade D according to CSA Z180.1).

Storage of Flammable Products

Solvents and paints must be stored in certified fireproof cabinets, in accordance with the National Fire Code of Canada. Maximum storage quantities are limited to 250 L per cabinet. Containers must be grounded during transfer of flammable liquids. Solvent-soaked rags must be placed in sealed metal containers.

Waste Disposal

Paint waste, solvents, and contaminated filters are hazardous waste under the Interprovincial and International Movement of Hazardous Waste Regulations (Environment Canada). They must be disposed of by an authorized carrier. Used solvents can be recycled through distillation. Wash water from water-based gun cleaning must be treated before discharge.

Common Pitfalls to Avoid

Summary

This chapter covers the full range of theoretical and practical knowledge required for the Red Seal exam in automotive refinishing technician. Mastery of calculations, adjustments, and safety standards is essential for passing the exam and practicing the trade safely.